Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. A mark is defined as contiguous region or area on the surface of the article which contrasts visually with the adjacent surface. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape and uniform color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks.
Of particular interest is creating marks on coated or painted articles. Articles made of metal or various types of plastics are often painted or otherwise covered in various industrial coatings to protect and change the appearance of the article's surface. Laser ablating the coating in particular patterns to remove the coating and reveal the surface of the article underneath is a desirable way to create a mark on article. Covering an article with two or more layers of coatings and laser ablating a first coating to reveal a second coating underneath is another desirable way to create marks. Marking a product by removing a coating with a laser to reveal the article underneath is discussed in US patent application no. 2008/0152859, inventor Masanori Nagai, published Jun. 26, 2008. This method depends upon the brightness of the coating being brighter than the surface of the article. Japanese patent application no. 03-150842, inventor Iwasaki Noboru, published Oct. 29, 1992, describes removing one or more coating layers with a laser to reveal a coating layer underneath.
One thing that these references have in common is that in order to remove a coating without removing materials beneath the coating layer being removed, the laser ablation threshold for the material being removed must be lower than the laser ablation threshold of the material underneath. Laser ablation threshold is the minimum energy required to cause removal of material. This removal may be ablative, where enough energy is put into the material by the laser to cause the material to disassociate into plasma, or thermal, where the material is essentially melted and vaporized, or a combination of the two. Related to the ablation threshold is the damage threshold. The damage threshold is the minimum laser energy required to cause an undesirable change in the appearance of the material. The damage threshold for materials is generally lower and sometimes much lower than the ablation threshold. We define damage as any undesirable change in the appearance of the materials that comprise the article or under lying coating following laser removal of the topmost layer.
FIG. 1 shows an exemplary prior art tool path for marking an article. Tool path refers to the sequence of locations on an article which will be exposed to laser radiation in order to create the mark. This laser radiation can be continuous wave (CW) or pulsed. In either case, the laser and optical system will have a laser beam, which is the optical path along which laser energy, either pulsed or CW, travels when the laser is energized to emit radiation. FIG. 1 shows an article 10, coated with an opaque coating 11. A shape 12 outlines the area where material is to be removed to form the mark. A tool path 13 is laid out for a laser to begin removing material at the start point 14. The laser beam is then moved in relation to the article 10 along the tool path 13, removing material until reaching the end point 16. This tool path is optimized in the sense that the tool path is configured to maximize the amount of time the laser spends actually removing material, as opposed to positioning the laser beam without cutting. FIG. 2 shows the results of removing material as shown in FIG. 1. The article 20, with a coating 21 has had the coating removed from the area of the mark 22, exposing material underneath 24, 26. In this case, the laser irradiance has been selected to optimize material removal rates for the portion of the “T” shape that forms the vertical portion 24. Irradiance is the rate at which laser energy is applied to the surface of the article per unit area and is measured in Watts/cm2. This irradiance causes damage or undesirable appearance to the other portions of the “T” shape 26, resulting in unacceptable appearance of the mark. The prior art solution to this problem is to reduce slow down the movement of the laser with respect to the article or reduce the irradiance, both of which reduce throughput and are hence undesirable.
What is desired but undisclosed by the art is a reliable and repeatable method for removing material that does not damage under laying materials in cases where the ablation threshold for the material to be removed is close to or lower than the damage threshold of the under laying material, or where the damage threshold changes because of previous laser processing. What is needed then is a method for reliably and repeatably creating marks having a desired appearance on coated articles using a laser to remove a layer of coating without causing undesired damage to the under laying materials while maintaining acceptable system throughput.